Binding of Cysteine Synthase to the STAS Domain of Sulfate Transporter and Its Regulatory Consequences

The sulfate ion (SO₄²⁻) is transported into plant root cells by SO₄²⁻ transporters and then mostly reduced to sulfide (S²⁻). The S²⁻ is then bonded to O-acetylserine through the activity of cysteine synthase (O-acetylserine (thiol)lyase or OASTL) to form cysteine, the first organic molecule of the SO₄²⁻ assimilation pathway. Here, we show that a root plasma membrane SO₄²⁻ transporter of Arabidopsis, SULTR1;2, physically interacts with OASTL. The interaction was initially demonstrated using a yeast two-hybrid system and corroborated by both in vivo and in vitro binding assays. The domain of SULTR1;2 shown to be important for association with OASTL is called the STAS domain. This domain is at the C terminus of the transporter and extends from the plasma membrane into the cytoplasm. The functional relevance of the OASTL-STAS interaction was investigated using yeast mutant cells devoid of endogenous SO₄²⁻ uptake activity but co-expressing SULTR1;2 and OASTL. The analysis of SO₄²⁻ transport in these cells suggests that the binding of OASTL to the STAS domain in this heterologous system negatively impacts transporter activity. In contrast, the activity of purified OASTL measured in vitro was enhanced by co-incubation with the STAS domain of SULTR1;2 but not with the analogous domain of the SO₄²⁻ transporter isoform SULTR1;1, even though the SULTR1;1 STAS peptide also interacts with OASTL based on the yeast two-hybrid system and in vitro binding assays. These observations suggest a regulatory model in which interactions between SULTR1;2 and OASTL coordinate internalization of SO₄²⁻ with the energetic/metabolic state of plant root cells.     

Enhancement of Nitrate Uptake and Growth of Barley Seedlings by Calcium under Saline Conditions

The effect of Ca²⁺ on NO₃⁻ assimilation in young barley (Hordeum vulgare L. var CM 72) seedlings in the presence and absence of NaCl was studied. Calcium increased the activity of the NO₃⁻ transporter under saline conditions, but had little effect under nonsaline conditions. Calcium decreased the induction period for the NO₃⁻ transporter under both saline and nonsaline conditions but had little effect on its apparent K_m for NO₃⁻ both in the presence and absence of NaCl. The enhancement of NO₃⁻ transport by Ca²⁺ under saline conditions was dependent on the presence of Ca²⁺ in the uptake solution along with the salt, since Ca²⁺ had no effect when supplied before or after salinity stress. Although Mn²⁺ and Mg²⁺ enhanced NO₃⁻ uptake under saline conditions, neither was as effective as Ca²⁺. In longer studies, increasing the Ca²⁺ concentration in saline nutrient solutions resulted in increases in NO₃⁻ assimilation and seedling growth.     

Transporter-Mediated Uptake of 2-Chloro- and 2-Hydroxybenzoate by Pseudomonas huttiensis Strain D1

We investigated the mechanisms of uptake of 2-chlorobenzoate (2-CBa) and 2-hydroxybenzoate (2-HBa) by Pseudomonas huttiensis strain D1. Uptake was monitored by assaying intracellular accumulation of 2-[UL-ring-¹⁴C]CBa and 2-[UL-ring-¹⁴C]HBa. Uptake of 2-CBa showed substrate saturation kinetics with an apparent K_m of 12.7 ± 2.6 μM and a maximum velocity (V_max) of 9.76 ± 0.78 nmol min⁻¹ mg of protein⁻¹. Enhanced rates of uptake were induced by growth on 2-CBa and 2-HBa, but not by growth on benzoate or 2,5-di-CBa. Intracellular accumulations of 2-CBa and 2-HBa were 109- and 42-fold greater, respectively, than the extracellular concentrations of these substrates and were indicative of uptake mediated by a transporter rather than driven by substrate catabolism (“metabolic drag”). Results of competitor screening tests indicated that the substrate range of the transporter did not include other o-halobenzoates that serve as growth substrates for strain D1 and for which the metabolism was initiated by the same dioxygenase as 2-CBa and 2-HBa. This suggested that multiple mechanisms for substrate uptake were coupled to the same catabolic enzyme. The preponderance of evidence from tests with metabolic inhibitors and artificial electrochemical gradients suggested that 2-CBa uptake was driven by ATP hydrolysis. If so, the 2-CBa transporter would be the first of the ATP binding cassette type implicated in uptake of haloaromatic acids.     

Early effects of salinity on nitrate assimilation in barley seedlings

The effect of NaCl and Na₂SO₄ salinity on NO₃⁻ assimilation in young barley (Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO₃⁻ transporter was affected very little; the major effect of the salts was on its activity. Both Cl⁻ and SO₄²⁻ salts severely inhibited uptake of NO₃⁻. When compared on the basis of osmolality of the uptake solutions, Cl⁻ salts were more inhibitory (15-30%) than SO₄²⁻ salts. At equal concentrations, SO₄²⁻ salts inhibited NO₃⁻ uptake 30 to 40% more than did Cl⁻ salts. The absolute concentrations of each ion seemed more important as inhibitors of NO₃⁻ uptake than did the osmolality of the uptake solutions. Both K⁺ and Na⁺ salts inhibited NO₃⁻ uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity.

Unlike NO₃⁻ uptake, NO₃⁻ reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO₃⁻ in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO₃⁻ reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO₃⁻ transporter, rather than that of the NO₃⁻ reduction system, may be a critical factor to plant survival during salt stress.

     

atp Mutants of Escherichia coli Fail To Grow on Succinate Due to a Transport Deficiency

Escherichia coli atp mutants, which lack a functional H⁺-ATPase complex, are capable of growth on glucose but not on succinate or other C₄-dicarboxylates (Suc⁻ phenotype). Suc⁺ revertants of an atp deletion strain were isolated which were capable of growth on succinate even though they lack the entire H⁺-ATPase complex. Complementation in trans with the yhiF gene suppressed the growth of the Suc⁺ mutants on succinate, which implicates the yhiF gene product in the regulation of C₄-dicarboxylate metabolism. Indeed, when the E. coli C₄-dicarboxylate transporter (encoded by the dctA gene) was expressed in trans, the Suc⁻ phenotype of the atp deletion strain reverted to Suc⁺, which shows that the reason why the E. coli atp mutant is unable to grow aerobically on C₄-dicarboxylates is insufficient transport capacity for these substrates.     

Bioinformatic and Expression Analyses of Genes Mediating Zinc Homeostasis in Nostoc punctiforme

Zinc homeostasis was investigated in Nostoc punctiforme. Cell tolerance to Zn²⁺ over 14 days showed that ZnCl₂ levels above 22 μM significantly reduced cell viability. After 3 days in 22 μM ZnCl₂, ca. 12% of the Zn²⁺ was in an EDTA-resistant component, suggesting an intracellular localization. Zinquin fluorescence was detected within cells exposed to concentrations up to 37 μM relative to 0 μM treatment. Radiolabeled ⁶⁵Zn showed Zn²⁺ uptake increased over a 3-day period, while efflux occurred more rapidly within a 3-h time period. Four putative genes involved in Zn²⁺ uptake and efflux in N. punctiforme were identified: (i) the predicted Co/Zn/Cd cation transporter, putative CDF; (ii) the predicted divalent heavy-metal cation transporter, putative Zip; (iii) the ATPase component and Fe/Zn uptake regulation protein, putative Fur; and (iv) an ABC-type Mn/Zn transport system, putative zinc ZnuC, ZnuABC system component. Quantitative real-time PCR indicated the responsiveness of all four genes to 22 μM ZnCl₂ within 3 h, followed by a reduction to below basal levels after 24 h by putative ZIP, ZnuC, and Fur and a reduction to below basal level after 72 h by putative CDF efflux gene. These results demonstrate differential regulation of zinc transporters over time, indicating a role for them in zinc homeostasis in N. punctiforme.     

Intracellular pH Regulation in Cultured Astrocytes from Rat Hippocampus : II. Electrogenic Na/HCO3 Cotransport

In the preceding paper (Bevensee, M.O., R.A. Weed, and W.F. Boron. 1997. J. Gen. Physiol. 110: 453–465.), we showed that a Na⁺-driven influx of HCO₃ ⁻ causes the increase in intracellular pH (pH_i) observed when astrocytes cultured from rat hippocampus are exposed to 5% CO₂/17 mM HCO₃ ⁻. In the present study, we used the pH-sensitive fluorescent indicator 2′,7′-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) and the perforated patch-clamp technique to determine whether this transporter is a Na⁺-driven Cl-HCO₃ exchanger, an electrogenic Na/HCO₃ cotransporter, or an electroneutral Na/HCO₃ cotransporter. To determine if the transporter is a Na⁺-driven Cl-HCO₃ exchanger, we depleted the cells of intracellular Cl⁻ by incubating them in a Cl⁻-free solution for an average of ∼11 min. We verified the depletion with the Cl⁻-sensitive dye N-(6-methoxyquinolyl)acetoethyl ester (MQAE). In Cl⁻-depleted cells, the pH_i still increases after one or more exposures to CO₂/HCO₃ ⁻. Furthermore, the pH_i decrease elicited by external Na⁺ removal does not require external Cl⁻. Therefore, the transporter cannot be a Na⁺-driven Cl-HCO₃ exchanger. To determine if the transporter is an electrogenic Na/ HCO₃ cotransporter, we measured pH_i and plasma membrane voltage (V_m) while removing external Na⁺, in the presence/absence of CO₂/HCO₃ ⁻ and in the presence/absence of 400 μM 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS). The CO₂/HCO₃ ⁻ solutions contained 20% CO₂ and 68 mM HCO₃ ⁻, pH 7.3, to maximize the HCO₃ ⁻ flux. In pH_i experiments, removing external Na⁺ in the presence of CO₂/HCO₃ ⁻ elicited an equivalent HCO₃ ⁻ efflux of 281 μM s⁻¹. The HCO₃ ⁻ influx elicited by returning external Na⁺ was inhibited 63% by DIDS, so that the predicted DIDS-sensitive V_m change was 3.3 mV. Indeed, we found that removing external Na⁺ elicited a DIDS-sensitive depolarization that was 2.6 mV larger in the presence than in the absence of CO₂/ HCO₃ ⁻. Thus, the Na/HCO₃ cotransporter is electrogenic. Because a cotransporter with a Na⁺:HCO₃ ⁻ stoichiometry of 1:3 or higher would predict a net HCO₃ ⁻ efflux, rather than the required influx, we conclude that rat hippocampal astrocytes have an electrogenic Na/HCO₃ cotransporter with a stoichiometry of 1:2.     

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus

Scenedesmus cells grown on high CO₂, when adapted to air levels of CO₂ for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO₂ from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O₂ evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO₂ pump with K_0.5 for O₂ evolution of less than 5 micromolar CO₂ was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO₃⁻ concentrating process between pH 8 to 10 lowered the K_0.5 (DIC) from 7300 micromolar HCO₃⁻ in high CO₂ grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO₃⁻ pump was inhibited by vanadate (K_i of 150 micromolar), as if it involved an ATPase linked HCO₃⁻ transporter. The CO₂ pump was formed on low CO₂ by high-CO₂ grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO₃⁻ pump was partially activated on low CO₂ within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO₃⁻ pump at pH 9 had requirements similar to the activation of the CO₂ pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO₂-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO₂ pump and in some algae an additional vanadate-sensitive plasmalemma HCO₃⁻ transporter for a HCO₃⁻ pump.     

ATP-binding Cassette Subfamily C Member 5 (ABCC5) Functions as an Efflux Transporter of Glutamate Conjugates and Analogs

The ubiquitous efflux transporter ABCC5 (ATP-binding cassette subfamily C member 5) is present at high levels in the blood-brain barrier, neurons, and glia, but its in vivo substrates and function are not known. Using untargeted metabolomic screens, we show that Abcc5^−/− mice accumulate endogenous glutamate conjugates in several tissues, but brain in particular. The abundant neurotransmitter N-acetylaspartylglutamate was 2.4-fold higher in Abcc5^−/− brain. The metabolites that accumulated in Abcc5^−/− tissues were depleted in cultured cells that overexpressed human ABCC5. In a vesicular membrane transport assay, ABCC5 also transported exogenous glutamate analogs, like the classic excitotoxic neurotoxins kainic acid, domoic acid, and NMDA; the therapeutic glutamate analog ZJ43; and, as previously shown, the anti-cancer drug methotrexate. Glutamate conjugates and analogs are of physiological relevance because they can affect the function of glutamate, the principal excitatory neurotransmitter in the brain. After CO₂ asphyxiation, several immediate early genes were expressed at lower levels in Abcc5^−/− brains than in wild type brains, suggesting altered glutamate signaling. Our results show that ABCC5 is a general glutamate conjugate and analog transporter that affects the disposition of endogenous metabolites, toxins, and drugs.     

Modeling of Environmental Effects in Genome-Wide Association Studies Identifies SLC2A2 and HP as Novel Loci Influencing Serum Cholesterol Levels

Genome-wide association studies (GWAS) have identified 38 larger genetic regions affecting classical blood lipid levels without adjusting for important environmental influences. We modeled diet and physical activity in a GWAS in order to identify novel loci affecting total cholesterol, LDL cholesterol, HDL cholesterol, and triglyceride levels. The Swedish (SE) EUROSPAN cohort (N _SE = 656) was screened for candidate genes and the non-Swedish (NS) EUROSPAN cohorts (N _NS = 3,282) were used for replication. In total, 3 SNPs were associated in the Swedish sample and were replicated in the non-Swedish cohorts. While SNP rs1532624 was a replication of the previously published association between CETP and HDL cholesterol, the other two were novel findings. For the latter SNPs, the p-value for association was substantially improved by inclusion of environmental covariates: SNP rs5400 (p _{SE,unadjusted} = 3.6×10⁻⁵, p _SE,adjusted = 2.2×10⁻⁶, p _{NS,unadjusted} = 0.047) in the SLC2A2 (Glucose transporter type 2) and rs2000999 (p _{SE,unadjusted} = 1.1×10⁻³, p _SE,adjusted = 3.8×10⁻⁴, p _{NS,unadjusted} = 0.035) in the HP gene (Haptoglobin-related protein precursor). Both showed evidence of association with total cholesterol. These results demonstrate that inclusion of important environmental factors in the analysis model can reveal new genetic susceptibility loci.     

Haploinsufficiency of the Ammonia Transporter Rhcg Predisposes to Chronic Acidosis: Rhcg IS CRITICAL FOR APICAL AND BASOLATERAL AMMONIA TRANSPORT IN THE MOUSE COLLECTING DUCT*

Ammonia secretion by the collecting duct (CD) is critical for acid-base homeostasis and, when defective, causes distal renal tubular acidosis (dRTA). The Rhesus protein RhCG mediates NH₃ transport as evident from cell-free and cellular models as well as from Rhcg-null mice. Here, we investigated in a Rhcg mouse model the metabolic effects of Rhcg haploinsufficiency, the role of Rhcg in basolateral NH₃ transport, and the mechanisms of adaptation to the lack of Rhcg. Both Rhcg^+/+ and Rhcg^+/− mice were able to handle an acute acid load, whereas Rhcg^−/− mice developed severe metabolic acidosis with reduced ammonuria and high mortality. However, chronic acid loading revealed that Rhcg^+/− mice did not fully recover, showing lower blood HCO₃⁻ concentration and more alkaline urine. Microperfusion studies demonstrated that transepithelial NH₃ permeability was reduced by 80 and 40%, respectively, in CDs from Rhcg^−/− and Rhcg^+/− mice compared with controls. Basolateral membrane permeability to NH₃ was reduced in CDs from Rhcg^−/− mice consistent with basolateral Rhcg localization. Rhcg^−/− responded to acid loading with normal expression of enzymes and transporters involved in proximal tubular ammoniagenesis but reduced abundance of the NKCC2 transporter responsible for medullary accumulation of ammonium. Consequently, tissue ammonium content was decreased. These data demonstrate a role for apical and basolateral Rhcg in transepithelial NH₃ transport and uncover an incomplete dRTA phenotype in Rhcg^+/− mice. Haploinsufficiency or reduced expression of RhCG may underlie human forms of (in)complete dRTA.     

Intracellular pH Regulation in Cultured Astrocytes from Rat Hippocampus : I. Role of HCO3?

We studied the regulation of intracellular pH (pH_i) in single cultured astrocytes passaged once from the hippocampus of the rat, using the dye 2′,7′-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) to monitor pH_i. Intrinsic buffering power (β_I) was 10.5 mM (pH unit)⁻¹ at pH_i 7.0, and decreased linearly with pH_i; the best-fit line to the data had a slope of −10.0 mM (pH unit)⁻². In the absence of HCO₃ ⁻, pH_i recovery from an acid load was mediated predominantly by a Na-H exchanger because the recovery was inhibited 88% by amiloride and 79% by ethylisopropylamiloride (EIPA) at pH_i 6.05. The ethylisopropylamiloride-sensitive component of acid extrusion fell linearly with pH_i. Acid extrusion was inhibited 68% (pH_i 6.23) by substituting Li⁺ for Na⁺ in the bath solution. Switching from a CO₂/HCO₃ ⁻-free to a CO₂/HCO₃ ⁻-containing bath solution caused mean steady state pH_i to increase from 6.82 to 6.90, due to a Na⁺-driven HCO₃ ⁻ transporter. The HCO₃ ⁻-induced pH_i increase was unaffected by amiloride, but was inhibited 75% (pH_i 6.85) by 400 μM 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), and 65% (pH_i 6.55–6.75) by pretreating astrocytes for up to ∼6.3 h with 400 μM 4-acetamide-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS). The CO₂/HCO₃ ⁻-induced pH_i increase was blocked when external Na⁺ was replaced with N-methyl-d-glucammonium (NMDG⁺). In the presence of HCO₃ ⁻, the Na⁺-driven HCO₃ ⁻ transporter contributed to the pH_i recovery from an acid load. For example, HCO₃ ⁻ shifted the plot of acid-extrusion rate vs. pH_i by 0.15–0.3 pH units in the alkaline direction. Also, with Na-H exchange inhibited by amiloride, HCO₃ ⁻ increased acid extrusion 3.8-fold (pH_i 6.20). When astrocytes were acid loaded in amiloride, with Li⁺ as the major cation, HCO₃ ⁻ failed to elicit a substantial increase in pH_i. Thus, Li⁺ does not appear to substitute well for Na⁺ on the HCO₃ ⁻ transporter. We conclude that an amiloride-sensitive Na-H exchanger and a Na⁺-driven HCO₃ ⁻ transporter are the predominant acid extruders in astrocytes.     

Halotolerant Cyanobacterium Aphanothece halophytica Contains a Betaine Transporter Active at Alkaline pH and High Salinity

Aphanothece halophytica is a halotolerant alkaliphilic cyanobacterium which can grow in media of up to 3.0 M NaCl and pH 11. This cyanobacterium can synthesize betaine from glycine by three-step methylation using S-adenosylmethionine as a methyl donor. To unveil the mechanism of betaine uptake and efflux in this alkaliphile, we isolated and characterized a betaine transporter. A gene encoding a protein (BetT_{A. halophytica}) that belongs to the betaine-choline-carnitine transporter (BCCT) family was isolated. Although the predicted isoelectric pH of a typical BCCT family transporter, OpuD of Bacillus subtilis, is basic, 9.54, that of BetT_{A. halophytica} is acidic, 4.58. BetT_{A. halophytica} specifically catalyzed the transport of betaine. Choline, γ-aminobutyric acid, betaine aldehyde, sarcosine, dimethylglycine, and amino acids such as proline did not compete for the uptake of betaine by BetT_{A. halophytica}. Sodium markedly enhanced betaine uptake rates, whereas potassium and other cations showed no effect, suggesting that BetT_{A. halophytica} is a Na⁺-betaine symporter. Betaine uptake activities of BetT_{A. halophytica} were high at alkaline pH values, with the optimum pH around 9.0. Freshwater Synechococcus cells overexpressing BetT_{A. halophytica} showed NaCl-activated betaine uptake activities with enhanced salt tolerance, allowing growth in seawater supplemented with betaine. Kinetic properties of betaine uptake in Synechococcus cells overexpressing BetT_{A. halophytica} were similar to those in A. halophytica cells. These findings indicate that A. halophytica contains a Na⁺-betaine symporter that contributes to the salt stress tolerance at alkaline pH. BetT_{A. halophytica} is the first identified transporter for compatible solutes in cyanobacteria.     

NO3 ?-induced pH Changes in Mammalian Cells : Evidence for an NO3?-H+ Cotransporter

The effect of NO₃ ⁻ on intracellular pH (pH_i) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO₃ ⁻ induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO₃ ⁻- induced pH change was not due to modulation of Na⁺/H⁺ exchange, since it was also observed in Na⁺/H⁺ antiport-deficient mutants. Though NO₃ ⁻ is known to inhibit vacuolar-type (V) H⁺-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A₁. NO₃ ⁻/HCO₃ ⁻ exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO₃ ⁻, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B.S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448– 11454). Accumulation of intracellular NO₃ ⁻, measured by the Greiss method after reduction to NO₂ ⁻, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO₃ ⁻ with H⁺ (or the equivalent counter-transport of NO₃ ⁻ for OH⁻). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO₃ ⁻ and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO₃ ⁻ and was accentuated by acidification of the extracellular space. The putative NO₃ ⁻-H⁺ cotransport was inhibited markedly by ethacrynic acid and by α-cyano-4-hydroxycinnamate, but only marginally by 4,4′-diisothiocyanostilbene-2,2′ disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO₃ ⁻-induced pH changes in mammalian cells may be related, though not identical, to the NO₃ ⁻-H⁺ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO₃ ⁻ with H⁺ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.     

Phosphate absorption rates and adenosine 5'-triphosphate concentrations in corn root tissue

The correlations between ATP concentration in corn (Zea mays) root tissue and the rate of phosphate absorption by the tissue have been examined. Experimental variation was secured with 2,4-dinitrophenol, oligomycin, mersalyl, l-ethionine, 2-deoxyglucose, N₂ gassing and inhibition of protein synthesis. It is concluded that ATP could be the energy source for potassium phosphate absorption, but only if the transport mechanism possesses certain properties: oligomycin-sensitivity; creation of a proton gradient susceptible to collapse by uncouplers; phosphate transport via a mersalyl-sensitive Pi⁻-OH⁻ transporter; good activity at energy charge as low as 0.4; short enzymatic half-life for the ATPase or phosphate transporter; a linked mechanism for K⁺-H⁺ exchange transport, possibly electrogenic.     

The Uptake of NO3?, NO2?, and NH4+ by Intact Wheat (Triticum aestivum) Seedlings : I. Induction and Kinetics of Transport Systems

The inducibility and kinetics of the NO₃⁻, NO₂⁻, and NH₄⁺ transporters in roots of wheat seedlings (Triticum aestivum cv Yercora Rojo) were characterized using precise methods approaching constant analysis of the substrate solutions. A microcomputer-controlled automated high performance liquid chromatography system was used to determine the depletion of each N species (initially at 1 millimolar) from complete nutrient solutions. Uptake rate analyses were performed using computerized curve-fitting techniques. More precise estimates were obtained for the time required for and the extent of the induction of each transporter. Up to 10 and 6 hours, respectively, were required to achieve apparent full induction of the NO₃⁻ and NO₂⁻ transporters. Evidence for substrate inducibility of the NH₄⁺ transporters requiring 5 hours is presented. The transport of NO₃⁻ was mediated by a dual system (or dual phasic), whereas only single systems were found for transport of NO₂⁻ and NH₄⁺. The K_m values for NO₃⁻, NO₂⁻, and NH₄⁺ were, respectively, 0.027, 0.054, and 0.05 millimolar. The K_m for mechanism II of NO₃⁻ transport could not be defined in this study as it exhibited only apparent first order kinetics up to 1 millimolar.     

High-Resolution NMR Reveals Secondary Structure and Folding of Amino Acid Transporter from Outer Chloroplast Membrane

Solving high-resolution structures for membrane proteins continues to be a daunting challenge in the structural biology community. In this study we report our high-resolution NMR results for a transmembrane protein, outer envelope protein of molar mass 16 kDa (OEP16), an amino acid transporter from the outer membrane of chloroplasts. Three-dimensional, high-resolution NMR experiments on the ¹³C, ¹⁵N, ²H-triply-labeled protein were used to assign protein backbone resonances and to obtain secondary structure information. The results yield over 95% assignment of N, H_N, CO, C_α, and C_β chemical shifts, which is essential for obtaining a high resolution structure from NMR data. Chemical shift analysis from the assignment data reveals experimental evidence for the first time on the location of the secondary structure elements on a per residue basis. In addition T _1Z and T₂ relaxation experiments were performed in order to better understand the protein dynamics. Arginine titration experiments yield an insight into the amino acid residues responsible for protein transporter function. The results provide the necessary basis for high-resolution structural determination of this important plant membrane protein.     

Potential-dependent anion transport in tonoplast vesicles from oat roots

Potential-dependent anion movement into tonoplast vesicles from oat roots (Avena sativa L. var Lang) was monitored as dissipation of membrane potentials (Δψ) using the fluorescence probe Oxonol V. The potentials (positive inside) were generated with the H⁺-pumping pyrophosphatase, which is K⁺ stimulated and anion insensitive. The relative rate of ΔΨ dissipation by anions was used to estimate the relative permeabilities of the anions. In decreasing order they were: SCN⁻ (100) > NO₃⁻ (72) = Cl⁻ (70) > Br⁻ (62) > SO₄²⁻ (5) = H₂PO₄⁻ (5) > malate (3) = acetate (3) > iminodiacetate (2). Kinetic studies showed that the rate of Δψ dissipation by Cl⁻ and NO₃⁻, but not by SCN⁻, was saturable. The K_m values for Cl⁻ and NO₃⁻ uptake were about 2.3 and 5 millimolar, respectively, suggesting these anions move into the vacuole through proteinaceous porters. In contrast to a H⁺-coupled Cl⁻ transporter on the same vesicles, the potential-dependent Cl⁻ transport was insensitive to 4,4′-diisothiocyano-2,2′-stilbene disulfonate. These results suggest the existence of at least two different mechanisms for Cl⁻ transport in these vesicles. The potentials generated by the H⁺-translocating ATPase and H⁺-pyrophosphatase were nonadditive, giving support to the model that both pumps are on tonoplast vesicles. No evidence for a putative Cl⁻ conductance on the anion-sensitive H⁺-ATPase was found.     

Sigma-1 receptor deficiency reduces MPTP-induced parkinsonism and death of dopaminergic neurons

Sigma-1 receptor (σ₁R) has been reported to be decreased in nigrostriatal motor system of Parkinson''s disease patients. Using heterozygous and homozygous σ₁R knockout (σ₁R^+/− and σ₁R^−/−) mice, we investigated the influence of σ₁R deficiency on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-impaired nigrostriatal motor system. The injection of MPTP for 5 weeks in wild-type mice (MPTP-WT mice), but not in σ₁R^+/− or σ₁R^−/− mice (MPTP-σ₁R^+/− or MPTP-σ₁R^−/− mice), caused motor deficits and ~40% death of dopaminergic neurons in substantia nigra pars compacta with an elevation of N-methyl-d-aspartate receptor (NMDAr) NR2B phosphorylation. The σ₁R antagonist NE100 or the NR2B inhibitor Ro25-6981 could alleviate the motor deficits and the death of dopaminergic neurons in MPTP-WT mice. By contrast, MPTP-σ₁R^+/− mice treated with the σ₁R agonist PRE084 or MPTP-σ₁R^−/− mice treated with the NMDAr agonist NMDA appeared to have similar motor deficits and loss of dopaminergic neurons as MPTP-WT mice. The pharmacological or genetic inactivation of σ₁R suppressed the expression of dopamine transporter (DAT) in substantia nigra, which was corrected by NMDA. The activation of σ₁R by PRE084 enhanced the DAT expression in WT mice or σ₁R^+/− mice. By contrast, the level of vesicular monoamine transporter 2 (VMAT2) in σ₁R^+/− mice or σ₁R^−/− mice had no difference from WT mice. Interestingly, MPTP-WT mice showed the reduction in the levels of DAT and VMAT2, but MPTP-σ₁R^−/− mice did not. The inactivation of σ₁R by NE100 could prevent the reduction of VMAT2 in MPTP-WT mice. In addition, the activation of microglia cells in substantia nigra was equally enhanced in MPTP-WT mice and MPTP-σ₁R^−/− mice. The number of activated astrocytes in MPTP-σ₁R^−/− mice was less than that in MPTP-WT mice. The findings indicate that the σ₁R deficiency through suppressing NMDAr function and DAT expression can reduce MPTP-induced death of dopaminergic neurons and parkinsonism.Parkinson''s disease (PD) is a neurodegenerative disorder characterized by motor symptoms, including bradykinesia and tremor, and a progressive loss of dopaminergic neurons in substantia nigra pars compacta (SNpc).^{1, 2} Sigma-1 receptor (σ₁R), previously named the opioid receptor sigma-1, is found primarily in motoneurons localized in the brainstem and spinal cord.³ The σ₁R is expressed in dopaminergic neurons and astrocytes.⁴ The σ₁R agonist PRE084 has been reported to exert neurorestorative effects on 6-hydroxydopamine (6-OHDA)-induced parkinsonism.⁴ Using positron emission tomography, the σ₁R-binding sites are found to be reduced in the brains of early-phase PD patients.⁵ However, the influence of σ₁R deficiency on the pathogenesis of PD has not yet been reported.Dopamine toxicity is involved in the etiology of PD.⁶ The σ₁R-binding sites on dopaminergic nerve terminals are involved in increasing dopamine release by enhancing N-methyl-d-aspartate receptors (NMDAr).⁷ The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) metabolized to 1-methyl-4-phenylpyridinium in glial cells selectively impairs dopaminergic neurons in SNpc through disrupting respiratory enzymes and causing oxidative damage.⁸ The dopamine transporter (DAT), a high-affinity transmembrane protein, is responsible for dopamine reuptake from the synaptic cleft and the transportation of 1-methyl-4-phenylpyridinium into dopaminergic nerve terminals.⁹ The σ₁R is co-expressed with DAT in dopaminergic neurons.⁴ Furthermore, the low density of DAT has been confirmed in the brains of PD patients.⁵The activation of σ₁R enhances the Ca²⁺ influx across NMDAr through increasing the phosphorylation of NR2B or the trafficking NMDAr to the plasma membrane.^{10, 11} The NMDAr NR2B inhibitor can attenuate MPTP- or 6-OHDA-induced parkinsonian symptoms and neurodegeneration.¹² The σ₁R deficiency has been demonstrated to reduce Aβ-induced neuronal cell death through suppressing NR2B phosphorylation.¹³ The inflammation is a predominant aspect of PD, manifested by glial activation with the expression of pro-inflammatory mediators.¹⁴ Sustained neuro-inflammation can exacerbate the degeneration of dopaminergic neurons.¹⁵ The blockade of σ₁R has been reported to inhibit methamphetamine-induced astrogliosis.¹⁶ Moreover, the 6-OHDA-induced spontaneous rotations or decline of dopaminergic fibers in σ₁R knockout mice seem to be less than those in wild-type (WT) mice.⁴ Paquette et al. reported that the blockade of σ₁R could attenuate abnormal involuntary movements induced by 6-OHDA.¹⁷In this study, we employed heterozygous and homozygous σ₁R knockout (σ₁R^+/− and σ₁R^−/−) mice to investigate the influence of σ₁R deficiency on MPTP-induced parkinsonism and death of dopaminergic neurons, and the underlying molecular mechanisms. Using the experimental PD models of MPTP-treated σ₁R^+/− mice and σ₁R^−/− mice, the present study provides in vivo evidence that the σ₁R deficiency through suppressing NMDAr function and DAT expression can attenuate MPTP-induced dopaminergic neurodegeneration and parkinsonism.